The present invention relates to a process for reaction injection molding (RIM) of two-colored molded articles having different properties, such as color, between the surface and inside portions using polyurethane.
Such a molding process, as described below, has been proposed in U.S. Pat. No. 5,628,944.
In a first step, the cavity of a mold is depressurized (evacuated).
In a second step, a first polyurethane material for the surface of the article is injected from a gate into the cavity under reduced pressure. The injected first polyurethane material is allowed to flow into the cavity after foaming under reduced pressure. It adheres in the form of a thin film over substantially the entire inner surface of the cavity due to break up of the foam, which forms a fine surface that is essentially free of air bubbles.
In a third step, a second polyurethane material for the inside of the article is injected from a gate into the cavity under a vacuum. The injected second polyurethane material flows into and fills the cavity where it reacts and is cured, which forms the inside of a two-colored molded article.
This two-colored molding process makes it possible to easily form a two-colored molded article in which a surface portion that contains a pigment covers an inside portion that contains no pigment. However, when the gate residue is cut away from the two-colored molded article, the non-colored inside is visible at the cut, which produces a poor appearance due to the conspicuous difference in color from the surface portion. In addition, when the first polyurethane material adhering to the inner surface of the cavity still has fluidity in the second step, the first polyurethane material adhered to the cavity surface, near the gate in particular, is carried away by the second polyurethane material, which reduces thickness of the first material when the second polyurethane material is injected in the third step. As a result, the surface at that site becomes transparent and the non-colored inside is visible. This results in a conspicuous coloration difference.
Therefore, a molding process has been proposed as described below (Japanese Unexamined Patent Publication No. Hei 9-183138). This process contains the steps listed below.
(1) A step of depressurizing the cavity of a mold;
(2) an initial step of injecting a small amount of a first polyurethane material containing a pigment into the cavity under reduced pressure to form the surface portion of a molded article;
(3) an intermediate step of injecting a second polyurethane material containing no pigment into the cavity under reduced pressure to form the majority of the inside of a molded article; and
(4) a final step of injecting a small amount of polyurethane material for the portion near the gate that contains a pigment into the cavity under reduced pressure to form the portion of the inside of the molded article near the gate.
Although the above-mentioned problems of color differences resulting from cutting away the gate residue and decreased thickness of the surface portion at sites near the gate are solved by this two-colored molding process, consideration has not been given to problems like those described below.
Molds used for RIM polyurethane molding are normally split molds. Since a gap exists between the facing surfaces of the upper and lower dies, a burr 100 is formed as shown in FIG. 18. Since this burr 100 has a three-layer cross sectional structure, in which a second, non-pigmented, polyurethane material 102 is between two layers of a first, pigmented, polyurethane material 101, when a finishing process is performed by cutting away the burr 100 along the broken line of FIG. 18(a), the non-colored second polyurethane material 102 is exposed in the form of a band as shown in FIG. 18(b) and FIG. 18(c). This produces a conspicuous color difference with the first polyurethane material 101 and flaws the appearance.
In addition, to prevent the non-colored second polyurethane material 102 from being seen through the first polyurethane material 101, it is necessary to increase the pigment concentration in the first polyurethane material 101. In FIG. 18(b), the diagonally hatched squares schematically illustrate pigment particles 103 that have a high density (high pigment concentration). In addition, since the thickness of the surface portion at sites near the gate decreases over a certain range, a considerable amount of polyurethane material is required for the portion near the gate, and it is also necessary to increase the pigment concentration in the polyurethane material for the portion near the gate. Thus, the required amount of pigment is high, which increases costs and accelerates the wear of equipment and jigs (such as mixing heads), depending on the type of pigment granules.
Therefore, it is an object of the present invention to provide a process for producing polyurethane molded articles that reduces the total amount of pigment used to reduce costs and reduce wear of equipment and jigs while preventing conspicuous color differences due to cutting away gate residue and thinning of the surface portion at sites near the gate.
The present invention provides a process for injecting a urethane material from a gate into a cavity of a mold and molding articles. In a first step, a surface layer is formed on the inner surface of the cavity with the mold closed. Next, a first urethane material containing a pigment is injected into the cavity. Then, a second urethane material containing more pigment than the first urethane material is injected into the cavity to produce a urethane molded article.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.